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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

140
ALASKAN GLACIER STUDIES
increase in curvature of the three prominent medial moraines and their extension to the right with the advance of the glacier down the fiord. These pictures also suggest an explanation of the lower bend in these medials in connection with the crowding down of the unsupported ice edge of the stagnant south tributary.
Comparison of the 1909 and 1910 photographs shows clearly that the 1910 ice front was an intermediate position between that of 1905 and that of 1909, the margins which receded during this period of four years of retreat not having yet advanced in 1910 to* the 1905 position. The lengthening of the medial moraines was also apparent and there seemed to be a thickening on the north side near the curve in the crevassed north tributary. By comparing these two photographs with the 1905 picture from the same site-it was evident that at the east end of the nunatak the crevassing which extends into the edge of the stagnant south tributary was more extensive in 1909 than in 1905 and less-extensive in 1910 than in 1909, as would be natural if the main crevassed glacier was thinned by ablation between 1905 and 1909 and thickened again by advance betweens 1909 and 1910.
Soundings in the fiord in 1910, discussed more fully in Chapter XI, showed that the depth of water about a thousand feet west of the ice front of Nunatak Glacier was 555
FIG. 11.    CBosar-SncTiON or NUNATAK GLACIER AND FIOBD. AND HORIZONTAL SCALE Tgn SAME.
VERTICAL
feet. A true scale cross-section of Nunatak Glacier when it was at that point (sometime between 1906 and 1909) is reproduced as Fig. 11 and shows (a) that the glacier was about 750 feet thick; (b) that as the portion above sea level was only 200 feet (two-sevenths of the thickness), the glacier could not possibly be afloat; (c) that the slopes of the fiord walls above and below sea level are not significantly different; (d) that the proportion of the glacial valley now occupied by the ice is much less than when the greater glacier overrode and rose high above the nunatak.
Nunatak Glacier from 1911 to 1913. In 1911 and 1912 the Nunatak Glacier was visited by a Boundary Survey party under the direction of N. J. Ogilvie. In 1911 they observed a slight continuation of the advance of the previous year. In 1912 the glacier had commenced to melt back again, having retreated an amount estimated to be a quarter mile.
In September, 1913, the junior author revisited the Nunatak Glacier with the International Geological Congress. It was evident that the tidal ice front was still retreating, the northern margin projecting farther west than the center of the glacier, but no precise measurements of the amount of diminution were obtained.
Relation to Earthquake. The advance which commenced in 1910 was a minor one, such as has affected Turner and Hubbard Glaciers several times since 1899, not a greater one, such as has come about in Galiano, Haenke, Atrevida, Lucia, Marvine, and